The idea for the bigger rubber washer was very good indeed.
I couldn't find a square ring, but got an o ring with 9mm internal diameter and a bigger outer diameter. And it seems to be working, as no bubbles come out of the plug during the soap test.

So I guess I can try to put the board in the water now, and pray the damage is permanently fixed .

What happens with the salt crystals remaining in the EPS? Do they produce vapor that could cause problems later on? A friend told me that he takes the salt out using vinegar, that he removes afterwards with a vacuum pump. Is it worth it to get into this kind of trouble?